Means for preventing radiointerference from electric motors



y 1930- A. .1. HASWELL 1,771,438

MEANS FOR PREVENTING RADIOINTERFERENCE FROM ELECTRIC MOTORS Filed Nov.26, 1928 2 Sheets-Sheet l INVENTOR BY AT TOR EY July 29, 1930. A; J,HASWELL 1,771,438

MEANS FOR PREVENTING RADIOINTERF'ERENCE FROM ELECTRIC MOTORS FiledjNov.26, 1928 2 Sheets-Sheet 2 I!" ATTORNEY HM \NVENTOR v Patented July 29,1930 UNITED STATES PATENT OFFICE ARTHUR JAMES HASWELL, 01' LONDON,ENGLAND, ASSIGNOB TO OTIS ELEVATOR COMPANY, OF NEW'YOBK, N. Y.

MEANS FOR PREVENTING BADIOINTEBFEBENGE FROM ELECTRIC MOTORS Applicationfiled November 26, 1928, Serial in). 821,914, and in Great BritainJanuary 18, 192 8.

- This invention relates to means for controlling or preventingelectrical interference with wireless telegraphic or telephonicinstallations from operatin dynamoelectric machinery such as electricmotors. The object of the present invention is to obviate or cure thistrouble and to provide means in association with electric motors orother dynamo-electric machines, which are adapted to eliminate the kindof interference.

across the brushes of the motor static condensers of relatively lar ecapacity, grouped as in series, the connection tween the condensers beinmaintained at earth potential, but it has en found in practice that thismethod is not successful or is indiilerently or only partiallysuccessful The invention comprises a method of preventin interferencewith wireless installations rom operating electric motors consistingbroadly in eliminating electric oscillations by insulating the motorframe from earth and connecting the brushes in closed circuit with themotor frame through a capacity. I I

According to the invention the electric motor or other machine isinsulated both from its bed plate and power coupling, that is, thecasing of the machine is electrically isolated from earth, andcondensers are connected across the brush holders and have a commonreturn connection taken back to the insulated frame of the machine; p

The commutation'of the armature should be adjusted to prevent sparkingor to reduce the same to a minimum, the brush rocker bein located in oras near as possible to the neutra zone. The brushes to be employedshould be 'or condenser banks C and C,

of low density, such as for example brushes of graphite or graphitecomposition, so as to reduce resistance at the point of brush contact toas low an amount as practicable.

A further feature of the invention consists in providing means forautomatically earthmg the frame of the electric motor or otherdynamo-electric machine to avoidaccidents when the motor is not workingor when inspection is desired. 1

The value of he condenser capacity in the circuit may be variedaccording to the particular characteristics of the motor to be treatedbut a capacity should be chosen which is adapted to tune the closedcircuit to the frame such that the oscillations are inaudible in thereceiving set. i I

In the accompanying drawings Figure 1 illustrates diagrammatically anelectric motor arranged according to an embodiment of the invention; f

Fi ure 2 is a diagram of the circuit in its aim ified form;

igure 3' illustrates di ammatically an electric motor and certain 0 itsassociated ap-' paratus arranged in accordance with the invention; and

Figure 4 and Figure 5 illustrate schematically alternative circuits forcausing the automatic operation of the grounding switch.

In carryingthe invention into efiect according to one method in whichit' is ap lied to an electric motor, the motor frame E Figurel) isinsulated from the bed late by suitable insulating material I. e brushesB and B are connected each to condensers respectively, which in thiscase are made u 0 parallel pairs of condensers in series. e condenseran!!! C and C are connected b the conducm tor L, and a common lead L omthe mid point L of this conductor is taken back to the motor carcass orcasing F as at L. In the common lead L. a condenser or condenser unitsC'may be introduced for'the purpose of as reducing or varying thecapacityof the closed circuit.

The condensers forming-the groups C, C are conveniently'supported on ablock of insulating material D and should be capable of 10a,

withstanding easily the electrical pressure of the supply. Thus, inapplying the invention to a direct-current ironclad motor of 4horsepower running on a 400 volt supply, the condensers chosen should becapable of withstanding 600 volts and have a capacity of the order oftwo microfarads each. The condenser C may be identical with the othercondensers. A high non-inductive resistance R, shown dotted in thedrawings, may be placed across the brushes of the motor to act as adamping resistance or to minimize the efiect of commutator ripple.

In Figure 2 the circuit is shown with the condenser omitted from thecommon lead. The frame F of the machine is assumed-to be insulated fromearth and the brushes B, B are connected through the condensers C C themidpoint L of the connection being taken back to a point L on the frameF. There need be no additional condenser in the common lead, if thecapacities of C and C, which are assumed equal, are chosen or selectedsuch that the oscillations in the closed circuit are not of a frequencyaudible in the receiving set.

Figure 3 diagrammatically illustrates an electric motor mounted with itsmotor frame F insulated from the grounded bed plate B by suitableinsulating material I, as in F igure 1, and also diagrammaticallyillustrates a manner by which the electric motor may be insulated fromits power coupling 11. As shown, this is effected by positioning a plateof insulating material 12 between the two flanges of the coupling, andby inserting suitably formed insulating bushes and washers 13 betweenthe coupling flanges and the coupling bolts. The motor frame F is thuselectrically isolated from earth.

In order to prevent accidents due to the motor frame becoming alive, aswitch S is provided for connecting the frame to earth when it isdesired to inspect the motor. The switch may be operated manually, or itmay be placed under automatic control, or both. In Figure 1 the switch Sis shown diagrammatically as a break in thegrounding circuit. Aconvenient mode of automatic control for the switch S is effected byactuating it by or in conjunction with a motor control switch. This isillustrated in Figure 3 wherein switch S is a set of contacts adapted tobe engaged by a bridge 16 of the electromagnetic switch 17. Theelectromagnetic switch 17 controls the operation of the motor by meansof its contact bridges in the motor circuits 18 and 19. As aresult ofthis arrangement, whenever the actuating coil of the electromagneticmotor control switch 17 is energized, the bridge 16 disengages from theS contactsto disconnect the motor frame F from the grounded motor bed B.When the actuating coil of the motor control switch is deenergized, thebridge drops back into engagement with the S contacts to ground themotor frame.

The switch S may be operated by means of a relay as a result of theoperation of the lighting switch for the room or area in which the motoris placed, so that anyone entering to inspect the motor, in switching onthe light, would automatically earth the motor frame. This manner ofautomatic control is illustrated in Figure 4, wherein the operation of aswitch 20 to cause the illumination of a bulb21 also causes theoperation of a relay 22. The relay 22 is provided with a contact bridge23, so that when the relay is operated the bridge 23 engages thecontacts S and connects the motor frame to ground.

Alternatively, the switch S may be automatically controlled by placing asmall relay in one of the control circuits of the installation. This isillustrated in Figure 5, wherein there is shown a relay 24 placed in acontrol circuit for the motor so that the relay is operated whenever thecontrol handle 25 is movedto start the motor. The relay is provided witha contact bridge 26 normally engaging the S contacts to ground the motorframe when the relay 24 is not operated and hence when the motor is notrunning. The operation of the relay 24 upon the starting of the motorcauses the bridge 26 to disengage the S contacts and thus to disconnectthe motor frame from the ground.

What is claimed is 1. Apparatus for preventing interference withwireless telephonic or telegraphic 1nstallations by an operatingelectric motor or dynamo-electric machine comprising, in combination,amachine having its frame insulated from earth, a connection forming aclosed circuit between the brushes and the frame and a condensercapacity in the closed circuit. I

2. Apparatus for preventing interference with wireless telephonic ortelegraphic 1nstallations by an operating electrromotor ordynamo-electric machine, comprising, in combination, a machine havingits frame 1nsulated from earth, a condenser capacity connected acrossthe brushes and a connectlon forming a closed circuit between saldcondenser capacityand the frame of themachine.

3. Apparatus for preventing interference with wireless telephonic ortelegraphic 1nstallations by an operating electriomotor or 1dynamo-electric machine comprising, 1n combination, a machine having itsframe 1nsulatedfromearth, a condenser capacity connected across thebrushes, :2. connection forming a closed circuit between the mid pointof said condenser ca acity and the frame of the machine, and a hnon-inductive resistance associated wit saidclosed c1rcuit.-- 4.Apparatus for preventing interference with wireless telephonic ortelegraphic installations by an operating electric motor ordynamo-electric machine comprising, in combination, means for insulatingthe 'frame of the machine from earth, a closed circuit between thebrushes and the frame, and a capacity in said closed circuit.

5. Apparatus for preventing interference with wireless telephonic ortelegraphic installations b an operating electric motor or dynamo-eectric machine comprising, in combination, a machine having its frameinsulated 'from earth, a plurality of condenser capacity units in seriesconnected across the brushes of said machine, a connection from a pointbetween the end condenser units to the machine frame, and condensercapacity inserted in said'connection.

6. Apparatus for preventing interference with wireless telephonic ortelegraphic installations by an operating electric motor or dnamo-electric machine comprising, in comb1nation, means for insulatingthe frame of the machine from earth, a connection from the brush of onepolarity to the frame, a capacit inserted in said connection, aconnection rom a brush of another polarity to the first namedconnection, and a capacity inserted in said second named connection.

7. Apparatus for preventing interference with wireless telephonic ortelegraphic installations by an operating electric motor comprising, incombination, means for insulating the motor frame from its support,means for insulating the armature of said motor from the shaftingmechanicall connected thereto, a plurality of condenser units in seriesconnected across the motor brushes, a connection from a point betweenthe end condenser units to the motor frame, and a condenser capacityinserted in said connec tion.

8. Apparatus for preventing interference with wireless telephonicortelegraphic installations by an operating electric motor ordynamo-electric machine comprising, in combination, means for insulatingthe structural parts of the machine from earth, a plurality of condensercapacity units in series. connected across the brushes of said machine,a connection from a point between the end-condenser units to the machineframe, condenser capacity inserted. in said connection, and a highnon-inductive resistance connected across the brushes of said machine.

-9. A -paratus for preventing interference second named connection, aconnection from said frame to earth, and a switch inserted in said\third named connection, said switch being adapted when closed to groundsaid frame.

10. Apparatus for preventin interference with wireless telephonic or teegraphic installations by an operating electric motor or dynamo-electricmachine comprising, in combination, means for insulating the frame ofthe machine from earth,-a closed circuit between the brushes and theframe, a capacity in said closed circuit, and means whereby the frame ofthe machine may be grounded.

11. Apparatus for preventing interference with wireless telephonic ortelegraphic installations by an operating electric motor ordynamo-electric machine comprising, in combination, means for insulatingthe frame of the machine from earth, a connection from the brush of onepolarity to the frame, a capacity insertedin said connection, aconnection from a brush of another polarity to the first namedconnection, a capacity inserted in said second named connection, acontrol switch for said machine, and means responsive to an operation ofsaid control switch to cause the grounding of said frame.

12. Apparatus for preventin interference with wireless telephonic or tee aphic installations by an operating electric motor or dynamo-electricmachine comprising, in combination, means for insulating the structuraldenser units to the machine frame, condenser capaclty inserted in saidconnection, and

means responsive to an incident in sto ping said machine to cause theconnection 0 said frame to earth and responsive to an incident instarting said machine for calls connection of said frame from eart Intestimony whereof I have signed in name to this specification. I

ARTHUR JAMES HASWELL.

the diswith wireless telephonic or telegraphic installations by anoperating electric motor or dynamo-electric machine comprising, incombination, means for insulating the frame of the machine from earth, aconnection from the brush of one polarity to'the frame, a capacityinserted in said connection, a connec tion from a brush of anotherpolarity to the first named connection, a capacity in'said

